The present invention relates generally to carburetor control systems for internal combustion engines and, more particularly, to an improved override speed control system for a carburetor of a small internal combustion engine.
In many uses of internal combustion engines, it is desirable that the engine be controlled so that its speed remains relatively constant under different loading conditions. For instance, in the use of the engine to power a lawn mower, it is desirable that the speed of the engine selected by an operator remain relatively constant whether the lawn mower encounters tall or short grass. This function is accomplished by means of three elements which are common to most engines of the type revealed herein. The three elements are a governor, a governor spring, and a speed control system. These three elements are interconnected with one another and with the carburetor throttle by a system of levers and wire links.
The governor generates a force which is proportional to the speed of the engine. The governor is connected to the carburetor throttle through a governor lever and governor link. The governor force biases the carburetor throttle toward closed position. The governor spring is connected to the governor lever and, when stretched to a length greater than its free length, it produces a force that biases the throttle lever toward open position.
The speed control system serves to modulate the length of the governor spring and thereby modulate its prevailing spring force. When the engine is not running, any force in the governor spring will pull the carburetor throttle to wide open position. As the engine accelerates, the governor force will reach a level that equals or exceeds the prevailing governor spring force. The governor will thereby overpower the governor spring and rotate the governor lever toward closed throttle position. As the governor lever rotates toward the closed throttle position, the governor spring suffers a slight extension of length and, therefore, a slight increase of its prevailing force. Eventually a spring length and corresponding force is reached that just equals the prevailing governor force. This equilibrium point establishes a carburetor throttle position that is appropriate to maintain the proper engine speed at the prevailing engine load. If the engine load increases, the engine will slow down slightly. This allows the governor spring to contract slightly in order to maintain equilibrium with the governor force thus increasing the throttle opening.
One override speed control system for the carburetor of a small internal combustion engine, such as used on a lawn mower, is disclosed in U.S. Pat. No. 4,517,942, issued to Pirkey et al., which is assigned to the assignee of the present invention. The control system of this patent basically includes a speed control lever, an intermediate speed override lever, and a throttle control of governor lever. The three levers are interconnected by a pair of extension springs, one of which extends between and connects the intermediate lever and the speed control lever spaced from it in one direction. The other extension spring extends between and connects the intermediate lever and the governor lever spaced from it in the opposite direction.
The speed control system maintains a constant engine speed for any setting of the speed control lever. When the speed control lever is in its last increment of movement, it actuates the choke valve of the carburetor. The intermediate lever is unresponsive to the control lever in its last increment of movement. Thus, the speed setting of the engine will remain constant irrespective of the choking action of the speed control lever. The governor maintains control over the engine speed under varying load conditions from the idle setting through the choke setting of the speed control lever. The setting of the speed control lever is selected by the operator through pivotal actuation of a hand lever, which is connected to the speed control lever by a control cable.
Speed adjustment is customarily accomplished by means of adjustment screws that limit the motion of the intermediate speed override lever. Override is accomplished by a torsion spring that biases the speed control lever and the intermediate lever against a common stop that establishes an angular relationship between the two levers. At the point of speed setting established by the speed set screw, the intermediate lever motion stops, but the speed control elver can continue in motion as the stop points separate and the torsion spring winds up. This provides override capability in only one direction, traditionally beyond the high speed set point.
It is desired to provide expanded override capability as well as a different arrangement of the components of the speed control system for more efficient override speed control action.